The present invention relates to a combustion control apparatus for internal combustion engines that includes a variable valve timing mechanism for adjusting the valve overlap of intake valves and exhaust valves and changes the combustion mode between stratified combustion and homogeneous combustion.
Japanese Unexamined Patent Publication No. 5-248277 discloses a combustion control apparatus that has fuel injectors located in combustion chambers of an engine. The apparatus changes the combustion mode of the engine between stratified combustion and homogeneous combustion according to the current running state of the engine. Specifically, when the engine is running with a relatively small load, the apparatus performs stratified combustion by injecting fuel in the vicinity of the spark plug in each combustion chamber at the end of the compression stroke. At this time, the fuel is concentrated about the spark plug. When the engine load is relatively high, the apparatus performs homogeneous charge combustion by homogeneously injecting fuel into each entire combustion chamber during the intake stroke.
The apparatus of the publication No. 5-248277 also includes a variable valve timing mechanism to adjust the valve overlap in accordance with the state of air-fuel mixture during the stratified and homogenous combustion modes. Particularly, during stratified combustion, the valve overlap is adjusted to create inner exhaust gas recirculation (EGR), which improves combustion and reduces NO.sub.x in the emission.
As described above, the apparatus of the publication optimizes the combustion state of the engine by adjusting the valve overlap in accordance with the current combustion mode referring to the current running state of the engine. However, this apparatus sometimes fails to control the valve overlap in accordance with the engine running state.
For example, the apparatus cannot control the valve overlap when the variable valve timing mechanism is not operating normally, that is, when the mechanism is malfunctioning or when the power supplied to the mechanism is insufficient. Also, the apparatus cannot control the valve overlap when the variable valve timing mechanism is not operating in accordance with the running state of the engine, that is, when a procedure for determining a referential position of the mechanism has not been executed.
In these cases, stratified combustion hinders the internal EGR control, which deteriorates the combustion of the engine. A reduced amount of EGR increases the amount of NO.sub.x in the emissions.